Spire permanently installed atop 1 World Trade Center

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One World Trade Center stands at its full height above the New York City skyline in this view from the Heights neighborhood of Jersey City, N.J., Friday, May 10, 2013. A 408-foot spire was set into place at the top of the structure Friday, making the building a symbolic 1,776 feet tall. (AP Photo/Julio Cortez)

May 10, 2013 2:22:00 PM PDT

Associated Press

NEW YORK - May 10, 2013 --

The silver spire topping One World Trade Center has been fully installed on the building's roof, bringing the iconic structure to its full, symbolic height of 1,776 feet.

The spire's installation was completed Friday morning, after pieces of it had been transported to the roof of the building last week. Construction workers below applauded the milestone.

The building is rising at the northwest corner of the site where the twin towers were destroyed in the Sept. 11, 2001, terror attacks. The area is well on its way to reconstruction with the 72-story Four World Trade Center and other buildings.

The 408-foot spire, weighing 758 tons, will serve as a world-class broadcast antenna. An LED-powered light emanating from it will be seen from miles away and a beacon will be at the top to ward off aircraft.

The addition of the spire, and its raising of the building's height to 1,776 feet, would make One World Trade Center the tallest structure in the U.S. and third-tallest in the world, although building experts dispute whether the spire is actually an antenna - a crucial distinction in measuring the building's height.

If it didn't have the spire, One World Trade Center would actually be shorter than the Willis Tower in Chicago, which stands at 1,451 feet and currently has the title of tallest building in the U.S., not including its own antennas.

The Council on Tall Buildings and Urban Habitat, a Chicago-based organization considered an authority on such records, says an antenna is something simply added to the top of a tower that can be removed. By contrast, a spire is something that is part of the building's architectural design.

The tower is slated to open for business in 2014.

Tenants include the magazine publisher Conde Nast, the government's General Services Administration and Vantone Holdings China Center, which will provide business space for international companies.